Artist’s impression of how Mars may have looked about four billion years ago, with an ocean occupying almost half of the northern hemisphere. (ESO/M Kornmesser)

Maps of the isotopic composition of water in the atmosphere of Mars have shown just how much of the planet’s water has been lost to space, and indicate an early ocean on Mars with more water than the Atlantic Ocean on Earth.

Lead author Geronimo Villanueva at NASA’s Goddard Space Flight Center and his team used infrared spectrographs on ground-based telescopes – the European Southern Observatory’s Very Large Telescope, the W M Keck Observatory and NASA’s Infrared Telescope Facility in Hawaii – to measure the relative proportions of hydrogen and deuterium in water at Mars. Ocean water on Earth has about 3200 molecules of H2O for every HDO molecule; martian water was lost to space through processes that preferentially affect the lighter molecule, leaving the water remaining on Mars much richer in HDO. The greater the water loss, the higher the ratio of HDO to H2O in the remaining water.

Villanueva and his team mapped Mars for six years (three martian years) and found surprising variations: microclimates and seasonal changes. They found that atmospheric water near the poles was enriched in HDO by a factor of seven compared to water on Earth, and polar ice enriched by a factor of eight. Together these suggest that the planet lost 6.5 times more water than is currently in the ice caps, making an ocean of at least 200 million km3 in the Noachian period. If this had occupied the low-lying Northern Plains, comparable in area to the Atlantic, it would have been more than 1.9km deep in places.

This work highlights the power of ground-based telescopes to study planetary processes. It also suggests that Mars may have had a much longer period as a wet and potentially habitable planet.

This research was published by Villanueva et al. in Science on 5 March.

This image is published in the April 2015 issue of Astronomy & Geophysics.